As described in U.S. Pat. No. 4,181,855 and other sources of the art, measurements associated with radioactivity and other events are commonly carried out with scintillation counters having a pair of detectors to produce a series of paired signals representative of the passage of particles passing through the counter tube between the detectors. With large scintillation counters used in time measurements associated with the passage of elementary particles at high velocities, the accuracy of the measurements are limited by the time uncertainty between the detector signals associated with the difference in time that the signals in the counter tube reach and trigger the detectors.
Additional details associated with scintillation counters and the time uncertainty factor may be provided by reference to FIG. 1. In FIG. 1, a scintillation counter is illustrated with detectors at each end to generate a pair of signals representative of the passage of a particle through the tube. With a series of particles passing through the tube at various points, the signals at each of the detectors will vary in order of time and will have a maximum uncertainty or difference equal to L/v or .DELTA.t.sub.o. These signals may be represented by the following: EQU t.sub.A =t.sub.o +x/v EQU t.sub.B =t.sub.o +[(L-x)/v] EQU t.sub.A +t.sub.B =2t.sub.o +L/v=2t.sub.o +.DELTA.t.sub.o
where "t.sub.A " and "t.sub.B " are time measurements at the first and second ends of the tube, "t.sub.o " is a constant associated with the time that the particle passes through the tube and the time for the signal to pass through the counter lightguides and photomultipliers, "L" is the length of the tube, "x" is the point along the tube where the particle passes, and "v" is the effective velocity for the light signal (generated by the passage of the particle) to propagate in the counter to reach the detectors. As the particles pass through the counter tube at various points along its length, a series of paired signals are generated at the two detectors representative of the series of individual particles or single events. When the particles pass through the tube at a distance "x" equal or less than L/2, the signal t.sub.1 at that end will be first or equal in order of time to the signal t.sub.2 at the other end. When "x" is greater than L/2, the signals t.sub.1 and t.sub.2 will be reversed respective to the terminals A and B and t.sub.1 and t.sub.2 will vary between the two detector outputs and will have a maximum uncertainty or difference in time of .DELTA.t.sub.o associated with the length of the counter tube. Particularly with high speed particles, the time uncertainty between the signals limits the usefulness of the data.
In addition to the maximum time uncertainty or difference between the signals t.sub.1 and t.sub.2 in each signal pair, the value of t.sub.2 will also vary between a maximum t.sub.2 equal to L/v and L/2v or between .DELTA.t.sub.o and .DELTA.t.sub.o /2 resulting in a second uncertainty. With elementary particles traveling at high velocities, reduction in these uncertainties is desirable to improve the usefulness of the output signals or data.
One object of this invention is the reduction in the maximum time uncertainty between a pair of signals representative of a single event. A second object is the reduction in the maximum time uncertainty of the second signal in time of a pair of signals representative of a single event. Another object is the improvement in usefulness of data generated by the passage of elementary particles through scintillation counters. A further object of the invention is the selection of signals first and second in the order of time from signal pairs with varying orders of time. An additional object of the invention is a logic circuit with AND and OR gates for time selection of the signals from detectors associated with scintillation counters. Other objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.
This invention provides a method and apparatus for reducing the maximum time uncertainty or difference .DELTA.t.sub.o between paired time signals t.sub.1 and t.sub.2 in a series of paired signals representative of a series of single events where the signals t.sub.1 and t.sub.2 vary between two terminals. The invention further provides a method and apparatus for reducing the maximum time uncertainty of the second signal t.sub.2 in a series of paired signals where t.sub.2 is dependent on the relationships t.sub.1 .ltoreq.t.sub.2 and t.sub.1 +t.sub.2 equals a constant. A maximum value (t.sub.max) for the signal t.sub.2 also usually equals L/v with t.sub.1 being at its minimum value (t.sub.min).
The apparatus of the invention includes input and output means, including a pair of input terminals for providing a series of paired signals t.sub.1 and t.sub.2 to the input terminals. At each terminal, the signals will vary between t.sub.1 and t.sub.2 where t.sub.1 .ltoreq.t.sub.2 and t.sub.1 +t.sub.2 equals a constant. The signals t.sub.1 and t.sub.2 vary within a predetermined limit of t.sub.min and t.sub.max whose difference is equal to .DELTA.t.sub.o. The apparatus includes circuit means with logic means interconnecting the input terminals to the output means for each stage or the entire circuit, and include means for adding a time delay to the signals t.sub.1 in the series to increase t.sub.min to t.sub.x where t.sub.min &lt;t.sub.x &lt;t.sub.max, and means for selecting signals between t.sub.x and t.sub.max whose maximum uncertainty is less than .DELTA.t.sub.o. The addition of time delay may be carried out either prior to or subsequent to the selection process. The time delay is equal to a fraction of .DELTA.t.sub.o with the value depending on the particular circuit. With a series circuit, the time delay preferably is equal to about .DELTA.t.sub.o /2. A single stage may provide significant reduction or several stages may be used. With a plurality of stages in a series, the time delay for each stage is reduced by a factor of two for each succeeding stage resulting in progressively reduced time uncertainties between the signals with the lower limit progressively approaching the upper limit t.sub.max.
The logic means includes first and second selecting means for selecting the first and second signals with the first selecting means including one or more OR gates and the second selecting means including one or more AND gates. In some embodiments as illustrated in the drawing, combinations of OR and AND gates are used for each selecting operation.
In addition to reducing the maximum time uncertainty .DELTA.t.sub.o of the signal pairs, the time delay at least in some embodiments also serves to increase the value of the second signal in those circuits where its maximum value is less than its original maximum value t.sub.max. By use of the invention, the maximum time uncertainty of signal pairs may be reduced by factors of 2, 4, 16 and values of 250-800 and above. In one embodiment (as generally illustrated by FIG. 3) with a .DELTA.t.sub.o equal to approximately 4.2 nsec., the maximum time uncertainty has been reduced to a value of approximately 70 psec. This and other embodiments are further discussed below in the detailed description.